Pertussis heat-labile toxin (PEHLT) is one of several toxins produced by Bordetalla pertussis. The biochemical and pharmacologic effects of PHELT and its relationship to clinical pertussis in humans is not known. PEHLT exhibits toxic activities when injected subcutaneously, intraperitoneally, or intravenously into rodents. We examined the gross and histophathologic features of a highly purified preparation of PHELT in mice. Highly purified PEHLT (200 pg) injected subcutaneously into newborn mice elicited an early acute inflammatory resonse in the superficial subcutaneous tissues and lower dermis followed by a mononuclear cell perivascular infliltrate, vascular congestion, dermal edema, extravasation of erythrocytes, and finally hemorrhage. Injection of the toxin intraperitoneally into 18-20 gm NIH general purpose mice resulted in thymic involution and a dose dependent decrease in splenic size. The changes in splenic size correlated with the histologic findings of necrosis of erythroid and myeloid precursors in the red pulp. At sublethal doses, PHELT also caused depletion of erythroid and myeloid elements in the bone marrow.